The present invention relates to an audio bridge using an echo canceller used in speech conferences etc.
In speech conferences, there has been used an audio bridge to add and synthesize speech signals transmitted from a plurality of conference attendants to transmit the synthesized speech signals to the respective conference attendants.
In such speech conferences, for the purpose of preventing a voice of the conference attendant himself from being fed back to a telephone receiver during conversation, a technique is employed to add only speech signals transmitted from other conference attendants except for the speech signal from the concerned conference attendant itself. Hitherto, a circuit arrangement using an echo canceller has been known as audio bridge.
For instance, a conference telephone system as shown in the Japanese Patent Laid-open No. 57-133754 comprises echo cancellers to cancel an echo signal fed back via a hybrid adapted for converting from 2-wire to 4-wire operation, audio bridge circuits, each being operative to add speech signals of channels except for the one assigned to itself to deliver the speech signals thus added to the receiver side of the conference attendant, and overload protecting circuits operative to control outputs of said audio bridge circuits so that overload condition does not occur in each receiving path of said echo cancellers and 4-wire transmission paths.
This conference telephone system can provide the following advantages: (1) There is no possibility that the transmission path is in overload condition. (2) Received sound volume is not lowered. (3) Echo loss is not Echo loss is not degraded, thus making it possible to prevent "singing". (4) Excellent speech quality can be obtained.
However, the drawback with such an audio bridge using the above-mentioned echo canceller is that echo cancellation or echo rejection level (referred to as ERL hereinafter) of the echo canceller varies with time, with the result that "singing" due to the variation of the cancellation with time cannot be avoided.